FATTY SUBSTANCES (GENERAL METHODS)
Percentage of oleic acid =
Percentage of SO
2 X °'°°4
x 100 = ->.-?
- x 100 =0-301
The acidity of a fat may also be expressed in degrees, which are dis-
tinguished as Kottstorfer and Burnstyn degrees. The former represent the
number of c.c. of normal KOH solution necessary to neutralise the free
acidity of 100 grams of a fat.
Burnstyn degrees (formerly used, especially to express the acidity of
lubricating oils) represent the number of c.c. of normal KOH required to
neutralise the acidity of 100 c.c. of oil, the test being carried out as follows :
100 c.c. of the oil are shaken with 100 c.c. of 90% alcohol ; when the latter
has separated and become clear, 25 c.c. of it are titrated with normal KOH,
tincture of turmeric or phenolphthalein being used as indicator. The
number of c.c. used, multiplied by 4, gives the Burnstyn acidity of the oil.1
Interconversion of the acidities expressed in different ways may be
effected by means of the following table :
Acid Number (mgrms. KOH per gram of substance).
Oleic Acid, o/ /o
Kottstorfer Degrees (c.c. N-KOH per 100 grams of substance).
The acid number of fatty substances is very variable. As a rule, fresh or
recently-prepared fats contain little or no free acid. With keeping, especially
if not well protected against the simultaneous action of air and light, the acidity
increases, slowly at first and more rapidly later. The acid number is of impor-
tance in judging edible oils and lubricants, neither of which should contain
more than certain limiting proportions of free acid.
8. Saponification Number
By saponification number is meant the number of milligrams of potassium
hydroxide (KOH) necessary to saponify completely i gram of a fatty sub-
stance. From this number the quantity of total acids, either free or com-
bined, in a fat may be deduced. The determination is made as follows, i
REAGENTS required are:
(i) Alcoholic caustic potash solution (about seminormal). Prepared
by dissolving about 32 grams of pure caustic potash in a little water and
making up to i litre with 95% alcohol free from fusel oil.2 The solution
1 The acidity of an oil determined in this way is always decidedly lower than that
determined by the preceding methods.
2 "When ordinary alcohol is used, the solution soon turns brown. Suitable alcohol
is obtained from good, commercial 95% alcohol by adding, and shaking with, powdered
potassium permanganate until a persistent red coloration is formed. The liquid is
left for some hours and distilled, in an apparatus provided with a dephlegmator, withbout